Coordination modes of tyrosinate-ligated catalase-type heme enzymes: magnetic circular dichroism studies of Plexaura homomalla allene oxide synthase, Mycobacterium avium ssp. paratuberculosis protein-2744c, and bovine liver catalase in their ferric and ferrous states

Molecular Basis for the Interaction of Catalase with d-Penicillamine: Rationalization of Some of Its Deleterious Effects

d-Penicillamine (d-Pen) is a sulfur compound used in the management of rheumatoid arthritis, Wilson's disease (WD), and alcohol dependence. Many side effects are associated with its use, particularly after long-term treatment. However, the molecular basis for such side effects is poorly understood. Based on the well-known oxidase activity of hemoproteins and the participation of catalase in cellular H₂O₂ redox signaling, we posit that d-Pen could inactivate catalase, thus disturbing H₂O₂ levels. Herein, we report on the molecular basis that could partly explain the side effects associated with this drug compound, and we demonstrate that it induces the formation of compound II, a temporarily inactive state of the enzyme, through two distinct mechanisms. Initially, d-Pen reacts with native catalase and/or iron metal ions, used to mimic non-heme iron overload observed in long-term treated WD patients, to generate thiyl radicals. These radicals partake in a futile redox cycle, thus producing superoxide radical anions O₂^•- and hydrogen peroxide H₂O₂. Then, either H₂O₂ unexpectedly reacts with reduced CAT-Fe(II) to produce compound II or both aforementioned reactive oxygen species intervene in compound II generation through compound I formation and then reduction. These findings support the evidence that d-Pen could perturb H₂O₂ redox homeostasis through transient but recurring catalase inactivation, which may in part rationalize some deleterious effects observed with this therapeutic agent, as discussed.

X-ray driven reduction of Cpd I of Catalase-3 from N. crassa reveals differential sensitivity of active sites and formation of ferrous state

Catalases are biotechnologically relevant enzymes because of their applications in food technology, bioremediation, and biomedicine. The dismutation of hydrogen peroxide occurs in two steps; in the first one, the enzyme forms an oxidized compound I (Cpd I) and in the second one, the enzyme is reduced to the ferric state. In this research work, we analyzed the reduction of Cpd I by X-ray radiation damage during diffraction experiments in crystals of CAT-3, a Large-Size Subunit Catalase (LSC) from Neurospora crassa. A Multi-Crystal Data collection Strategy was applied in order to obtain the Cpd I structure at a resolution of 2.2 Å; this intermediate was highly sensitive to X-ray and was easily reduced at very low deposited radiation dose, causing breakage of the Fe=O bond. The comparison of the structures showed reduced intermediates and also evidenced the differential sensitivity per monomer. The resting ferric state was reduced to the ferrous state, an intermediate without a previous report in LSC. The chemically obtained Cpd I and the X-ray reduced intermediates were identified by UV-visible microspectrometry coupled to data collection. The differential sensitivity and the formation of a ferrous state are discussed, emphasizing the importance of the correct interpretation in the oxidation state of the iron heme.

Sulfheme formation during homocysteine S-oxygenation by catalase in cancers and neurodegenerative diseases

Accumulating evidence suggests that abnormal levels of homocysteine are associated with vascular dysfunctions, cancer cell proliferation and various neurodegenerative diseases. With respect to the latter, a perturbation of transition metal homeostasis and an inhibition of catalase bioactivity have been reported. Herein, we report on some of the molecular bases for the cellular toxicity of homocysteine and demonstrate that it induces the formation of sulfcatalase, an irreversible inactive state of the enzyme, without the intervention of hydrogen sulfide. Initially, homocysteine reacts with native catalase and/or redox-active transition metal ions to generate thiyl radicals that mediate compound II formation, a temporarily inactive state of the enzyme. Then, the ferryl centre of compound II intervenes into the unprecedented S-oxygenation of homocysteine to engender the corresponding sulfenic acid species that further participates into the prosthetic heme modification through the formation of an unusual Fe(II) sulfonium. In addition, our ex cellulo studies performed on cancer cells, models of neurodegenerative diseases and ulcerative colitis suggest the likelihood of this scenario in a subset of cancer cells, as well as in a cellular model of Parkinson's disease. Our findings expand the repertoire of heme modifications promoted by biological compounds and point out another deleterious trait of disturbed homocysteine levels that could participate in the aetiology of these diseases.

High levels of homocysteine in cells are linked to pathological states. Here, the authors report that homocysteine inactivates catalase by modifying the heme group, impairing cellular redox homeostasis, and show that this modification occurs in cancer cells and in a cellular model of Parkinson's disease.

Corynebacterium diphtheriae HmuT: dissecting the roles of conserved residues in heme pocket stabilization

The heme-binding protein HmuT is part of the Corynebacterium diphtheriae heme uptake pathway and is responsible for the delivery of heme to the HmuUV ABC transporter. HmuT binds heme with a conserved His/Tyr heme axial ligation motif. Sequence alignment revealed additional conserved residues of potential importance for heme binding: R237, Y272 and M292. In this study, site-directed mutations at these three positions provided insight into the nature of axial heme binding to the protein and its effect on the thermal stability of the heme-loaded protein fold. UV-visible absorbance, resonance Raman (rR) and thermal unfolding experiments, along with collision-induced dissociation electrospray ionization mass spectrometry, were used to probe the contributions of each mutated residue to the stability of ϖ HmuT. Thermal unfolding and rR experiments revealed that R237 and M292 are important residues for heme binding. Arginine 237 is a hydrogen-bond donor to the phenol side chain of Y235, which serves as an axial heme ligand. Methionine 292 serves a supporting structural role, favoring the R237 hydrogen-bond donation, which elicits a, heretofore, unobserved modulating influence on π donation by the axial tyrosine ligand in the heme carbonyl complex, HmuT-CO.

Heme Binding by Corynebacterium diphtheriae HmuT: Function and Heme Environment

The heme uptake pathway (hmu) of Corynebacterium diphtheriae utilizes multiple proteins to bind and transport heme into the cell. One of these proteins, HmuT, delivers heme to the ABC transporter HmuUV. In this study, the axial ligation of the heme in ferric HmuT is probed by examination of wild-type (WT) HmuT and a series of conserved heme pocket residue mutants, H136A, Y235A, and M292A. Characterization by UV-visible, resonance Raman, and magnetic circular dichroism spectroscopies indicates that H136 and Y235 are the axial ligands in ferric HmuT. Consistent with this assignment of axial ligands, ferric WT and H136A HmuT are difficult to reduce while Y235A is reduced readily in the presence of dithionite. The FeCO Raman shifts in WT, H136A, and Y235A HmuT-CO complexes provide further evidence of the axial ligand assignments. Additionally, these frequencies provide insight into the nonbonding environment of the heme pocket. Ferrous Y235A and the Y235A-CO complex reveal that the imidazole of H136 exists in two forms, one neutral and one with imidazolate character, consistent with a hydrogen bond acceptor on the H136 side of the heme. The ferric fluoride complex of Y235A reveals the presence of at least one hydrogen bond donor on the Y235 side of the heme. Hemoglobin utilization assays showed that the axial Y235 ligand is required for heme uptake in HmuT.

A Catalase-related Hemoprotein in Coral Is Specialized for Synthesis of Short-chain Aldehydes: DISCOVERY OF P450-TYPE HYDROPEROXIDE LYASE ACTIVITY IN A CATALASE

In corals a catalase-lipoxygenase fusion protein transforms arachidonic acid to the allene oxide 8R,9-epoxy-5,9,11,14-eicosatetraenoic acid from which arise cyclopentenones such as the prostanoid-related clavulones. Recently we cloned two catalase-lipoxygenase fusion protein genes (a and b) from the coral Capnella imbricata, form a being an allene oxide synthase and form b giving uncharacterized polar products (Lõhelaid, H., Teder, T., Tõldsepp, K., Ekins, M., and Samel, N. (2014) PloS ONE 9, e89215). Here, using HPLC-UV, LC-MS, and NMR methods, we identify a novel activity of fusion protein b, establishing its role in cleaving the lipoxygenase product 8R-hydroperoxy-eicosatetraenoic acid into the short-chain aldehydes (5Z)-8-oxo-octenoic acid and (3Z,6Z)-dodecadienal; these primary products readily isomerize in an aqueous medium to the corresponding 6E- and 2E,6Z derivatives. This type of enzymatic cleavage, splitting the carbon chain within the conjugated diene of the hydroperoxide substrate, is known only in plant cytochrome P450 hydroperoxide lyases. In mechanistic studies using (18)O-labeled substrate and incubations in H2(18)O, we established synthesis of the C8-oxo acid and C12 aldehyde with the retention of the hydroperoxy oxygens, consistent with synthesis of a short-lived hemiacetal intermediate that breaks down spontaneously into the two aldehydes. Taken together with our initial studies indicating differing gene regulation of the allene oxide synthase and the newly identified catalase-related hydroperoxide lyase and given the role of aldehydes in plant defense, this work uncovers a potential pathway in coral stress signaling and a novel enzymatic activity in the animal kingdom.

Catalase (KatA) plays a role in protection against anaerobic nitric oxide in Pseudomonas aeruginosa

Pseudomonas aeruginosa (PA) is a common bacterial pathogen, responsible for a high incidence of nosocomial and respiratory infections. KatA is the major catalase of PA that detoxifies hydrogen peroxide (H₂O₂), a reactive oxygen intermediate generated during aerobic respiration. Paradoxically, PA displays elevated KatA activity under anaerobic growth conditions where the substrate of KatA, H₂O₂, is not produced. The aim of the present study is to elucidate the mechanism underlying this phenomenon and define the role of KatA in PA during anaerobiosis using genetic, biochemical and biophysical approaches. We demonstrated that anaerobic wild-type PAO1 cells yielded higher levels of katA transcription and expression than aerobic cells, whereas a nitrite reductase mutant ΔnirS produced ∼50% the KatA activity of PAO1, suggesting that a basal NO level was required for the increased KatA activity. We also found that transcription of the katA gene was controlled, in part, by the master anaerobic regulator, ANR. A ΔkatA mutant and a mucoid mucA22 ΔkatA bacteria demonstrated increased sensitivity to acidified nitrite (an NO generator) in anaerobic planktonic and biofilm cultures. EPR spectra of anaerobic bacteria showed that levels of dinitrosyl iron complexes (DNIC), indicators of NO stress, were increased significantly in the ΔkatA mutant, and dramatically in a ΔnorCB mutant compared to basal levels of DNIC in PAO1 and ΔnirS mutant. Expression of KatA dramatically reduced the DNIC levels in ΔnorCB mutant. We further revealed direct NO-KatA interactions in vitro using EPR, optical spectroscopy and X-ray crystallography. KatA has a 5-coordinate high spin ferric heme that binds NO without prior reduction of the heme iron (K_d ∼6 μM). Collectively, we conclude that KatA is expressed to protect PA against NO generated during anaerobic respiration. We proposed that such protective effects of KatA may involve buffering of free NO when potentially toxic concentrations of NO are approached.